The present invention relates to a resonant tunneling transistor used in a super high-speed element and a functional element.
A resonant tunneling phenomenon is known as a super high-speed phenomenon of 1 ps or less, and a resonant tunneling diode having an oscillation frequency of more than 600 GHz and using a negative resistance has already been reported. In addition, a resonant tunneling transistor obtained by directly adding the third electrode for controlling the resonance phenomenon to a resonant tunneling barrier structure has also been examined. The present invention relates to the latter resonant tunneling transistor.
FIG. 1 shows a typical arrangement of a conventional resonant tunneling transistor. The resonant tunneling transistor shown in FIG. 1 is arranged as follows. That is, on an n-type GaAs crystal semiconductor substrate 21, a semiconductor layer 22 consisting of n-type GaAs and serving as a collector layer, a semiconductor layer 23 serving as a first barrier layer against either of electrons and holes and consisting of, e.g., AlGaAs, a quantum well layer 24 serving as a base layer and consisting of GaAs doped with a p-type impurity, a semiconductor layer 25 serving as a second barrier layer against either of electrons and holes and consisting of, e.g., AlGaAs, and a semiconductor layer 26 serving as an emitter and consisting of n-type GaAs are sequentially grown by a normal molecular beam epitaxial method (MBE method) so as to form a collector electrode 27 and an emitter electrode 28 respectively connected to the semiconductor substrate 21 and the semiconductor layer 26. In addition, the semiconductor layer 26 is partially removed by etching to form a base electrode 29 connected to the quantum well layer 24.
FIGS. 2 and 3 show band structures corresponding to the stacked structure consisting of the collector layer 22 and the emitter layer 26. A power supply is connected between the collector electrode 27 and the emitter electrode 28 through a load such that the positive terminal of the power supply is connected to the collector electrode, and a bias voltage V.sub.BE is applied across the base electrode 29 and the emitter electrode 28 such that the base electrode is positive. When the bias voltage V.sub.BE is gradually increased, a Fermi level can be changed from the state in FIG. 2 to the state in FIG. 3. In these drawings, reference numeral 31 denotes a conduction band minimum; and 32, a valence band maximum. Reference symbol E.sub.f.sup.E denotes a Fermi level of an emitter layer; and E.sub.f.sup.B, a Fermi level of a base layer. Reference numeral 33 denotes a ground-state energy level of electrons in the quantum well layer 24; 34, a first excited-state level of the electrons in the quantum well layer 24; 35, a ground-state energy level of heavy holes in the quantum well layer 24; 36, an excited-state level of the heavy holes in the quantum well layer 24; 37, an ground-state energy level of light holes in the quantum well layer 24; 38, a first excited-state level of the light holes in the quantum well layer 24; and 39, a depletion layer formed in the emitter layer.
When the voltage V.sub.BE is increased to cause the ground-state energy level 33 to be lower than the Fermi level E.sub.f.sup.E of the emitter layer 26, electrons are injected from the emitter layer 26 into the base layer 24 by resonant tunneling. At this time, since most of the electrons flow into the collector, an amplifying operation similar to a normal bipolar transistor is obtained, thereby flowing a collector current Ic.
When the bias voltage is further increased, the ground-state energy level 33 becomes lower than the conduction band minimum of the emitter layer, thereby suppressing resonant tunneling.
When the bias voltage is further increased, the first excited-state level 34 becomes lower than the Fermi level E.sub.f.sup.E of the emitter layer, and electrons are injected from the emitter layer 26 into the base layer 24 by resonant tunneling. Since most of the electrons flow into the collector layer 22, a collector current is increased again. When the bias voltage is further increased, the first excited-state level 34 becomes lower than the conduction band minimum of the emitter layer 26 to suppress resonant tunneling, thereby decreasing the collector current.
As a result, as shown in FIG. 4, characteristics in which an emitter-collector current I.sub.C is periodically changed, i.e., I.sub.P1, I.sub.V1, I.sub.P2, and I.sub.V2, in accordance with an increase in emitter-base voltage V.sub.BE can be realized. Therefore, the resonant tunneling transistor having the above characteristics is examined to be used as a functional element having the functions of a parity generating circuit, a multi-value logical circuit, a multi-value memory circuit, an A/D converting circuit, and an oscillating element (Literature: F. Cappaso, Ed., "Physics of Quantum Electron Devices", Spring-Verlag, Berlin, Heidelberg, 1990).
The resonant tunneling transistor with the above arrangement, however, has the following various problems.
In a bias state in which resonant tunneling of electrons occurs, holes, especially light holes, flow from the base layer 24 to the emitter layer 26 through the second barrier layer 25 due to a tunneling phenomenon. This current corresponds to a forward current of a p-n junction, and the current is sharply increased in accordance with an increase in bias voltage. For this reason, an emitter injection efficiency is considerably decreased to disadvantageously decrease an amplification factor. In addition, when the resonant tunneling current of electrons is sharply increased, this current is canceled by the tunneling current of holes. Therefore, the negative transconductance disadvantageously disappears.
FIG. 5 shows the current-voltage characteristics of the emitter and base. In FIG. 5, a characteristic curve a represents a case wherein electrons in the emitter layer 26 are injected in the base layer 24 and holes in the base layer 24 are injected in the emitter layer 26, and a characteristic curve b represents a case wherein only electrons in the emitter layer 26 are injected in the base layer 24.
In order to solve the above problem, Frensley et al. proposed the following (Japanese Patent Application No. 62-111469: U.S. Ser. No. 07/768542, U.S. Ser. No. 07/825720). That is, a semiconductor having the energy gap of the forbidden band larger than that of the base layer 24 is used as an emitter (wide-gap emitter) layer 26' to cause band discontinuity between the emitter layer 26' and the base layer 24. An example of this proposal is shown in FIG. 6. In FIG. 6, reference symbols .DELTA.E.sub.C and .DELTA.E.sub.V denote band discontinuity values of a conduction band and a valence band, respectively. When a bias voltage is applied in an operative state, the band discontinuity value .DELTA.E.sub.V prevents holes located in the base layer 24 from flowing in the emitter region 26'. This method is similar to a known method for increasing an emitter injection efficiency by a hetero-junction bipolar transistor.
In this structure, however, to be described below, since the value .DELTA.E.sub.C necessarily generated with the value .DELTA.E.sub.V limits a level in the quantum well used for resonant tunneling of electrons to an excited-state level, another problem is posed as follows. That is, in resonant tunneling using an excited-state level, compared with resonant tunneling using a ground-state energy level, a ratio of a resonant tunneling current component to a non-resonant tunneling current component, i.e., a so-called peak-to-valley ratio (P/V ratio) is decreased. Although a detailed mechanism of the structure is not yet clarified, many experimental results pointing out this fact have been conventionally reported. Since the negative transconductance of the structure corresponds to the P/V ratio, an essential problem, i.e., a negative transconductance in such a resonant tunneling three-terminal element using an excited-state level, is posed.
The second problem posed when an excited-state level is used is a distortion of an output waveform. When electrons injected in the excited-state level flow into a collector layer, the electrons flow along two paths. The first path is a path in which the electrons are directly tunneled to the collector layer 22, and the second path is a path in which the electrons are once de-excited to a normal state by scattering of electrons in the base layer 24 and then tunneled to the collector layer 22. Tunneling times along the two paths are largely different from each other because the heights of the tunneling barriers of the two paths are different by a difference between the excited-state level and the ground-state energy level. For this reason, after a collector current flows along the first path, a collector current flows along the second path with a time lag. Therefore, the output current waveform is disadvantageously distorted.
Still another problem posed when an AlAs/GaAs hetero-junction is used together with the excited-state level will be described below. Since the effective mass (0.082 m.sub.e or less) of light holes of GaAs is substantially equal to the effective mass (0.067 m.sub.e) of electrons, the energies of the levels 33 and 37 in the quantum well are substantially equal to each other when the energies are measured from the bottom of the well. On the other hand, since it is known that the difference in the energy gap of a forbidden band is distributed to the discontinuity values of a conduction band and a valence band at a ratio of about 60:40, the value .DELTA.E.sub.C which is 1.5 times the value .DELTA.E.sub.V is generated For this reason, when tunneling of holes is prevented by the value .DELTA.E.sub.V, the value .DELTA.E.sub.C larger than the ground state of electrons is necessarily formed. For this reason, in a bias state wherein the ground-state energy level 33 is equal to the conduction band minimum of the emitter layer 26, the thickness of the barrier layer is actually increased due to a depletion layer on the emitter side, and a large resonant tunneling current is not obtained. Therefore, in order to obtain an actual resonant current, a bias voltage required for eliminating the depletion layer on the emitter side must be applied to cause resonance with the excited-state level so as to supply an emitter current.
In the above description, the AlAs/GaAs hetero-junction is used. However, even when a hetero-junction of other Group III-V compound semiconductors is used, the distribution state of band discontinuity and a relationship between electrons and light holes are not largely changed. For this reason, it is very difficult to solve the above problems by changing the materials.
As another method of suppressing holes from flowing into the emitter layer 26, the concentration of light holes in the base layer 24 is decreased. In a quantum well, the level of light holes is higher than that of heavy holes. For this reason, all the holes can serve as heavy holes by decreasing a Fermi level. However, in this method, the hole concentration has an upper limit, and a base resistance is disadvantageously increased. The base resistance is an important factor for defining the high-frequency operation of an element. More specifically, it is apparent from the analogy of a bipolar transistor that a maximum oscillation frequency f.sub.max is considerably decreased.
As another method of rising the ground-state energy level of light holes compared with the Fermi level in the quantum well layer 24, there is a method using a structure in which the width of the quantum layer 24 is decreased. However, in this structure, since a p-type impurity doped in the base layer is necessarily diffused in the emitter layer, an emitter injection efficiency is decreased, and a current gain is decreased. In addition, a negative transconductance disadvantageously decreased or disappears.
As described above, in the conventionally proposed structures, a large negative transconductance and a decrease in distortion of an output waveform cannot be realized without an increase in base resistance.